1. Field of the Invention
This invention relates to a steel sheet plated with Zn, Zn alloy such as Zn-Ni, Zn-Fe, Zn-Mg, Zn-Al, or the like, Al or with multi-layers of these metals which is excellent in cationic electrodeposition coating properties, press workability and corrosion resistance and which is also weldable.
2. Description of Prior Art
Recently, organic thin film multicoated steel sheets of less than 10 .mu.m thick have been increasingly used for improvement of productivity in the car industry, the appliance industry, etc. On the other hand, in order to meet the demand, various organic multicoated steel sheets and coating compositions have been developed in the steel industry and the paint industry. However, the conventional coating compositions have not sufficiently met the various quality requirements for organic multicoated steel sheets, for example, those in press working, electrodeposition, spot welding, etc.
That is, the zinc-rich coating compositions proposed in Japanese Patent KOKOKU (Post-Exam. Publn.) Nos. 24230/70 and 6882/72 can be electrodeposited, but are still insufficient in press workability, corrosion resistance and weldability. Furthermore, the coating compositions containing conductive pigment proposed in Japanese Patent KOKOKU (Post-Exam. Publn.) Nos. 44569/77 and 19706/83 and Japanese Patent KOKAI (Laid-Open) Nos. 138758/83 and 79138/76 have remarkably improved corrosion resistance in zinc-plated steel sheets and further improved weldability, but since they contain relatively large particle pigments such as zinc dust, metallic powder, metallic carbide, metallic phosphide, etc., the electrodeposited film is very irregular and lacks smoothness and the sheets are poor in press moldability and are limited in positions for use.
Furthermore, coating compositions containing no conductive pigment and capable of being electrodeposited in the form of thin film of 0.3-3 .mu.m are proposed in Japanese Patent KOKAI (Laid-Open) Nos. 33192/85, 224174/83 and 174879/85. Although organic composite silicate films formed therefrom and steel sheets coated therewith are improved in corrosion resistance, weldability and press moldability, they still suffer from the problems of irregularities on the electrodeposited surface, especially the problem of smoothness of the surface which varies greatly due to microvariation of the thickness of the organic composite silicate film and smooth electrodeposited film cannot be obtained.
As mentioned above, the conventionally proposed techniques are all insufficient to meet the present quality requirements for organic multicoated steel sheets, especially in adhesiveness of the electrodeposited film in wet and corrosive condition and smoothness of the surface.